Temperature dependence of far-infrared absorption in GaAs

The temperature dependence of the far-infrared absorption in GaAs has been measured. The results show that both two- and three-phonon difference processes contribute to the absorption. The two-phonon part of the absorption is dominated by processes which involve optical and longitudinal-acoustic phonons (O-LA) rather than longitudinal- and transverse-acoustic phonons (LA-TA). Since density-of-states arguments imply that the absorption from LA-TA should be an order of magnitude larger than O-LA the implication is that there is a considerable difference in the coupling coefficients. This result is consistent with the approximation selection rules proposed by Nedoluha.